1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device, particularly to a technique of improving withstanding operating voltage.
2. Description of the Related Art
FIG. 5 is a sectional view describing a conventional semiconductor device.
In FIG. 5, symbol 51 refers to a first conductive P type semiconductor substrate. On the substrate 51, a gate electrode 53 is formed through a gate oxide film 52, and a source-drain region of a one-sided LDD (Lightly Doped Drain) structure is formed adjacent to the gate electrode 53. That is, this semiconductor device has a source-drain region of the one-sided LDD structure. A high concentration (N+ type) source region 55 is formed at the source region side adjacent to the gate electrode 53, a low concentration (N− type) drain region 54 is formed at the drain region side adjacent to said gate electrode 53, and a high concentration (N+ type) drain region 56 is formed in the low concentration drain region 54.
As described above, in the semiconductor device of the one-sided LDD structure in which a high voltage is applied only to the drain region side, the high concentration drain region 56 is surrounded by the low concentration drain region 54 to defuse concentration of electric field in the drain region side, as mentioned above. However, in the source region side, only the high concentration source region 55 exists.
Even the semiconductor device having such a structure is needless to take its structure as a particular problem with regard to static withstanding voltage. However, at operation, the following problem occurs.
That is, in a bipolar structure consisting of a source region (emitter region), a substrate (base region), and a drain region (collector region), injection efficiency of carrier is good because a high concentration source region 55 is exposed in emitter region, so that the bipolar structure can be made easily by a little substrate current Isub.
That is, since current gain β in the bipolar structure with a one-sided LLD structure is high, the drain withstanding voltage during operation decreases compared with a semiconductor device of a double-sided LDD structure.
Here, in order to improve the drain withstanding voltage during operation, the substrate current Isub needs to be decreased. That is, the electric field must be made weak.
However, when an impurity concentration of the entire low concentration drain region 54 is decreased in order to decrease the substrate current Isub, the substrate current Isub has a double hump structure having two peaks ((1) and (2)) as voltage Vg increases, as shown in FIG. 6.
When the low concentration drain region 54 is further decreased, the first peak (1) of the substrate current Isub is low so that the drain withstanding voltage at low Vgs improves. However, the second peak (2) of the substrate current Isub is comparatively high so that drain withstanding voltage at high Vgs decreases.
Conversely, when the entire impurity concentration of the low concentration drain region 54 is high, a single peak at a certain voltage Vgs appears and the drain withstanding voltage at high Vgs decreases, as shown in FIG. 6. However, there is a problem that the drain withstanding voltage at low Vgs can not withstand.
Thus, when the entire impurity concentration of the low concentration drain region 54 is changed uniformly, the change can not overcome the trade-off relationship of the drain withstanding voltage at low Vgs and the drain withstanding voltage at high Vgs.
Although current gain β decreases and the withstanding voltage withstands decidedly by adopting a double-sided LDD structure, the device has distance (L) of a drift region similar to the drain side shown in FIG. 5. In this instance, the on-resistance increases and the driving ability decreases because a usual LDD structure is adopted at the source side, although the withstanding voltage at the source side is not needed.